Method for controlled free radical polymerization or copolymerization of ethylene under high pressure in the presence of an initiator-controller

ABSTRACT

The present invention relates to a process for the radical polymerization or copolymerization of ethylene under high pressure in the presence of at least one polymerization initiating-controlling compound capable of providing, by decomposition under the polymerization or copolymerization conditions: 
     at least one initiating free radical (Z) which carries at least one site for initiating the (co)polymerization; and 
     at least one stable free radical (SFR) which carries at least one site exhibiting the stable radical state and which is stable under the polymerization conditions, 
     with, in total, as many initiating sites as sites exhibiting the stable radical state.

The present invention relates to a process for the controlled radicalpolymerization or copolymerization of ethylene under high pressure inthe presence an initiator-controller.

The polymerization under high pressure of ethylene or itscopolymerization under high pressure with comonomers which cancopolymerize by the radical route results in a large variety of productswhich have numerous applications, among which may be mentioned bases foradhesives, in particular hot melt adhesives, bituminous binders,wrapping films, coextrusion binders, moulded items, and the like.

Processes for the polymerization of ethylene at high temperatures andpressures by means of free radical initiators have been known for a longtime. Ethylene polymers are obtained by homopolymerizing ethylene or bycopolymerizing it with at least one other comonomer in a polymerizationsystem which operates continuously under pressures of the order of 50MPa to 500 MPa and at temperatures of between 50 and 300° C. Thepolymerization is carried out in continuous tubular reactors or stirredautoclaves in the presence of initiators and optionally of transferagents. The polymers are subsequently separated from the volatilesubstances after their departure from the reactor in separators.

It is known that the polymerization of ethylene in the presence or inthe absence of comonomers can result in reaction runaways (see, forexample, Chem. Eng. Proc., 1998, 37, 55-59). These runaways arereflected by a very marked rise in the temperature and in the pressureand thus by bursting of the safety devices of the plant. Consequently,the runaway must result in undesired shutdowns in production. The aim isthus to avoid these shutdowns as far as possible and one means for doingthis is to carefully control the flow rates of the reactants enteringthe reactor, in particular the flow rate of the source of radicals, thatis to say of the initiator. This is because the injection of anexcessively large amount of radicals results in a localized runaway inone of the regions of the reactor, which runaway subsequently spreadsvery quickly to the whole of the reactor. There thus exists a content ofradicals not to be exceeded in order not to result in the runaway of thepolymerization.

However, it is generally known that radical polymerizations can becontrolled using stable free radicals, this control making it possiblein particular to obtain polymers exhibiting narrow molecular massdistributions. Thus it is that United States Patent U.S. Pat. No.5,449,724 discloses a radical polymerization process which consists inheating, at a temperature of approximately 40° C. to approximately 500°C. and under a pressure of approximately 50 MPa to 500 MPa, a mixturecomposed of a free radical initiator, of a stable free radical and ofethylene, in order to form a thermoplastic resin which has a molecularmass distribution of approximately 1.0 to approximately 2.0.

Furthermore, it is known, by International Patent Application WO99/03894, to control the radical polymerization of monomers by the use,as (co)polymerization initiators, of specific alkoxyamines, thesemonomers being styrene, substituted styrenes, conjugated dienes,acrolein, vinyl acetate, anhydrides of (alkyl)acrylic acids, salts of(alkyl)acrylic acids, (alkyl)acrylic esters and alkyl-acrylamides.Ethylene is not mentioned as monomer. This polymerization is carried outunder low pressure and at a temperature of between 50 and 180° C.,preferably between 80 and 150° C., control of the reaction no longerbeing possible beyond 180° C. In other words, such a process could notwork for the (co)polymerization of ethylene under high pressure, inwhich (co)polymerization the temperature conditions generally exceed1800° C. This process furthermore exhibits the limitation according towhich the polymers obtained have low molecular masses (not exceeding 15000 in the examples).

In seeking to improve the known process for the controlled radical(co)polymerization of ethylene under high pressure, the ApplicantCompany has now discovered that, if use is made in (co)polymerization ofan initiator-controller capable of providing at least one initiatingfree radical and at least one stable free radical, more specifically afree radical which is stable under the specific temperature conditionsdeployed in this high-pressure (co)polymerization, the latter iscontrolled under particularly favourable conditions while alsocontrolling the reaction stability. The preferred initiators-controllersof the present invention, which will be described below, constitute afamily of compounds which is not recommended according to WO 99/03894.It was therefore not obvious to thus control the high pressure(co)polymerization of ethylene, with greater effectiveness than with theuse of an initiator and of a stable free radical, which are introducedseparately, and with the observation, also surprising, that the(co)polymerization of ethylene takes place at a markedly greater rate.Furthermore, with the process of the invention, there is no limitationon the molecular masses of the (co)polymers obtained.

In addition, another consequence of the present invention is that, inthe case where the initiator-controller chosen is such that it providesan initiating free macroradical, block copolymers are produced in whichat least one of the blocks comprises ethylene as constituent. In pointof fact, ethylene copolymers prepared under high pressure currently haverandom structures and it has not been possible to date to obtain suchblock copolymers having an ethylene-based block. It is well known thatthe structure of block copolymers can result in markedly betterphysicochemical properties than random copolymers. The present inventionthus makes it possible to achieve the production of novel materialshaving novel properties.

The subject-matter of the present invention is thus first a process forthe radical polymerization or copolymerization of ethylene under highpressure in the presence of at least one polymerizationinitiating-controlling compound capable of providing, by decompositionunder the polymerization or copolymerization conditions:

at least one initiating free radical (Z) which carries at least one sitefor initiating the (co)polymerization; and

at least one stable free radical (SFR) which carries at least one siteexhibiting the stable radical state and which is stable under thepolymerization conditions,

with, in total, as many initiating sites as sites exhibiting the stableradical state.

In other words, when the initiator-controller dissociates, it produces,in the medium, as many initiating sites as stable radical sites. In thesimplest case, the initiator-controller is such that it dissociates togive one initiating free radical and one stable free radical, the tworadicals being monofunctional. Use may also be made ofinitiators-controllers which dissociate to give an n-functionalinitiating free radical and n monofunctional stable free radicals, orvice versa. Examples of various initiators-controllers are shown below.

The growing (co)polymer is thus positioned between the “initiating” partand the “controlling” part constituted by the stable free radical SFR.

The present invention thus involves the formation of a stable freeradical. A stable free radical should not be confused with free radicalswith a fleeting lifetime (a few milliseconds), such as the free radicalsresulting from the usual polymerization initiators, such as peroxides,hydroperoxides and initiators of the azo type. The free radicals whichinitiate polymerization tend to accelerate the polymerization. Incontrast, stable free radicals generally tend to slow down thepolymerization. It may be generally said that a free radical is stablewithin the meaning of the present invention if it is not apolymerization initiator and if, under the operating conditions of thepresent invention, the mean lifetime of the radical is at least fiveminutes. During this mean lifetime, the molecules of the stable freeradical continually alternate between the radical state and the state ofa group bonded to a polymer chain via a covalent bond resulting from acoupling reaction between a radical centered on an oxygen atom and aradical centered on a carbon atom. Of course, it is preferable for thestable free radical to exhibit good stability throughout the duration ofits use in the context of the present invention. Generally, a stablefree radical can be isolated in the radical state at ambienttemperature.

“Initiating ” Part

In accordance with a first embodiment, the choice is made, asinitiating-controlling compound, of a compound capable of providing atleast one monofunctional radical Z chosen from those of the formulae(Ia) or (Ib) or (Ic):

in which:

R¹, R², R³, R⁴, R⁵, R⁶ and R⁷ each independently represent:

optionally substituted C₁₋₂₄ alkyl;

optionally substituted C₁₋₂₄ aryl;

it also being possible for R¹, R², R³, R⁴, R⁵ and R⁶ to eachindependently denote a hydrogen atom.

As examples of this first embodiment, the choice is made, asinitiating-controlling compound, of a compound capable of providing atleast one monofunctional radical Z chosen from those of the formulae(Ia₁), (Ia₂) or (Ia₃):

with n=0 or an integer from 1 to 23;

with R⁸ representing hydrogen, methyl or ethyl; and

with:

R⁹, R¹⁰, R¹¹, R¹² and R¹³ each independently representing alkyl, aryl orhalogen; and

R¹⁴ representing alkyl or aryl.

In accordance with a second embodiment, the choice is made, asinitiating-controlling compound, of a compound capable of providing aradical Z of formula (II):

Z¹—(PM)¹—[(PM)²]^(•)  (II)

in which:

Z¹ represents the initiating fragment of a radical initiator;

(PM)¹ represents a polymer block formed by living radical polymerizationor copolymerization of at least one monomer which can polymerize by theradical route in the presence of an initiator which produces freeradicals Z^(1•); and

(PM)², the presence of which is optional, represents another polymerblock, other than (PM)¹, formed by living radical polymerization orcopolymerization of at least one monomer which can polymerize by theradical route in the presence of the initiator Z¹-(PM)^(1•).

Mention may be made, as examples of (PM)¹ and (PM)² blocks, of those ofthe formulae:

where:

T and U each independently represent hydrogen or a substituted orunsubstituted C₁₋₁₀ alkyl residue;

V and W each independently represent hydrogen, substituted orunsubstituted alkyl, substituted or unsubstituted aryl, —COOH, —COOR¹⁵,—CN, —CONH₂, —CONHR¹⁶, —CONR¹⁷R¹⁸,

or —OR²⁰, R¹⁵ to R²⁰ each independently representing substituted orunsubstituted alkyl or substituted or unsubstituted aryl; and

n denotes the degree of polymerization, which can in particular range upto 10 000.

This second embodiment thus relates to the use ofmacroinitiators-controllers, the “macro-initiator” part of which isprepared in a known way by living radical (co)polymerization under highpressure (for example>100 MPa), in the case where ethylene participatesin the preparation of at least one of the blocks (T=U=V=W=H), or underlow pressure, in the contrary case.

In accordance with a third embodiment, the choice is made, asinitiating-controlling compound, of a compound capable of providing apolyfunctional radical Z carrying a plurality of initiating sites of

type. The functionality can vary between 2 and 10, preferably beingbetween 2 and 4.

Mention may be made, by way of example, of a compound capable ofproviding a polyfunctional radical Z of formula:

“Controlling” Part

The choice is advantageously made, as initiating-controlling compound,of a compound capable of providing at least one nitroxyl stable freeradical comprising at least one═N—O^(•) group.

The stable free radical or radicals is/are chosen in particular fromnitroxide radicals, that is to say comprising the═N—O^(•) group, inparticular from those of the formulae (IIIa), (IIIb) or (IIIc):

in which:

R′¹ to R′³, R′⁵ to R′⁸ and R′¹³ and R′¹⁴ each independently represent:

(a) a hydrogen atom;

(b) a halogen atom, such as chlorine, bromine or iodine;

(c) a saturated or unsaturated and linear, branched or mono- orpolycyclic hydro-carbonaceous group which can be substituted by at leastone halogen;

(d) an ester group —COOR′¹⁵ or an alkoxyl group —OR′¹⁶, R′¹⁵ and R′¹⁶representing a hydrocarbonaceous group as defined in point (c) above;

(e) a polymer chain which can, for example, be a poly(alkylmethacrylate) or poly(alkyl acrylate) chain, such as poly(methylmethacrylate), a polydiene chain, such as polybutadiene, or a polyolefinchain, such as polyethylene or polybutadiene but preferably being apolystyrene chain;

R′⁴ has the meanings defined in points (a), (b), (c), (d) and (e) above;

R′⁹ to R′¹², which are identical or different, have the meanings definedin points (a) to (e) above and can in addition represent a hydroxylgroup or an acid group, such as —COOH or —SO₃H;

it being possible for R′³ and R′⁴ to be connected to one another and, inthe case where R′⁴ represents a —CR″¹R″²R″³ residue (R″¹ to R″³ havingwithout distinction the meanings of R′¹ to R′³), it being possible forR′³ to be connected to R″³, to form a heterocycle comprising thenitrogen atom of

it being possible for the said heterocycle to be saturated orunsaturated, to comprise, in the ring, at least one other heteroatomand/or at least one

group and also to comprise a saturated or unsaturated fused ring;

it being possible for two from R′¹ to R′³, R′⁵ and R′⁶, R′⁷ and R′⁸, R′⁹and R′¹⁰, R′¹¹ and R′¹², R′⁶ and R′⁹, R′⁸ and R′¹¹, R′¹³ and R′¹⁴ and,in the case where R′⁴ represents a —CR″¹R″²R″³ residue, R′³ and R″³independently to be connected to one another to form, with the carbonatom which carries them, a saturated or unsaturated ring or heterocycle;

u is a non zero integer, for example from 1 to 18.

Mention may be made, as examples of hydrocarbonaceous groups as definedin point (c) above, of those having from 1 to 20 carbon atoms, such aslinear, branched or cyclic alkyl radicals and aryl radicals, for examplephenyl or naphthyl, and radicals comprising at least one aromatic ringwhich can be substituted, for example by a C₁-C₄ alkyl radical, such asaralkyl radicals, for example benzyl.

Mention may in particular be made of nitroxide radicals comprising asequence,of formula:

in which the R_(L) radical exhibits a molar mass of greater than 15. Themonovalent R_(L) radical is said to be in the β-position with respect tothe nitrogen atom of the nitroxide radical. The remaining valences ofthe carbon atom and of the nitrogen atom in the formula (1) can bebonded to various radicals, such as a hydrogen atom or ahydrocarbonaceous radical, such as an alkyl, aryl, or aralkyl radical,comprising from 1 to 10 carbon atoms. It is not excluded that the carbonatom and the nitrogen atom in the formula (1) should be connected to oneanother via a bivalent radical, so as to form a ring. However, theremaining valences of the carbon atom and of the nitrogen atom of theformula (1) are preferably bonded to monovalent radicals. The R_(L)radical preferably exhibits a molar mass of greater than 30. The R_(L)radical can, for example, have a molar mass of between 40 and 450. TheR_(L) radical can also comprise at least one aromatic ring, as for thephenyl radical or the naphthyl radical, it being possible for the latterto be substituted, for example by an alkyl radical comprising from 1 to4 carbon atoms.

A specific family of nitroxide radicals which can be envisaged inaccordance with the present invention is that of the nitroxide radicalsof formula (IIIa) in which R′³ and R′⁴ (or R′³ and R″³) are connected toone another and which are chosen in particular from:

where:

R^(a) to R^(k) and R^(m) independently have the meanings given for R′⁹to R′¹², it being possible for R^(a) and R^(b) and R^(e) and R^(f) to beidentical or different when they are carried by different carbon atoms;

r has the value 2 or 3 or 4;

s a non zero integer, in particular from 1 to 10;

t has the value 0, 1 or 2.

The following may furthermore be indicated, as specific examples ofnitroxide radicals:

2,2,5,5-tetramethyl-1-pyrrolidinyloxy (generally sold under the tradename PROXYL):

3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (commonly known as3-carboxy-PROXYL);

2,2,6,6-tetramethyl-1-piperidinyloxy (commonly known as TEMPO):

4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (commonly known as4-hydroxy-TEMPO);

4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy (commonly known as4-methoxy-TEMPO):

4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (commonly known as4-oxo-TEMPO);

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, represented bythe formula:

(sold under the trade name “CXA 5415” by “Ciba Specialty Chemical”);

N-tert-butyl-1-phenyl-2-methylpropyl nitroxide:

N-tert-butyl-1-(2-naphthyl)-2-methylpropyl nitroxide.

In a particularly preferred way, however, use is made of an alkoxyamineas initiating-controlling compound and more particularly of analkoxyamine for which the nitrogen atom of the nitroxide group formspart of a C₅₋₁₂ ring, the other atoms of which are generally carbonatoms.

Mention may also particularly be made of alkoxyamines, the nitroxide ofwhich does not decompose to more than 50% over 2 hours at 180° C. under200 MPa (2 000 bar) in heptane.

The alkoxyamines are known compounds or compounds the manufacture ofwhich has been described in the literature. Reference may be made, interalia, to Macromolecules, 1996, 29, 5245-5254, to Macromolecules, 1996,29, 7661-7670, and to French Patent Application No. 99-04405 of 8 Apr.1999 on behalf of the Applicant Company.

Specific examples of initiators-controllers of the present invention arethe following:

(2,2,6,6-tetramethyl-1-piperidinyloxyhexane)

In accordance with the present invention, the ratio of theinitiating-controlling compound/monomer(s) is generally within the rangefrom 0.0001% to 10% by weight, in particular within the range from0.0005% to 5% by weight.

Furthermore, the (co)polymerization of the present invention isgenerally carried out under a pressure of 150 to 300 MPa, in particularof 150 to 250 MPa, and at a temperature of 100 to 300° C., in particularof 180 to 250° C.

There is no limitation with regard to the molecular masses of the(co)polymers obtained according to the invention. According to thepolymerization or copolymerization conditions and in particular theduration, the temperature or the degree of conversion of monomer topolymer or copolymer, it is possible to prepare products of differentmolecular masses. In particular, in the case of the polymerization ofethylene, the process of the invention is carried out at a temperature,a pressure and a duration which are sufficient for the polyethyleneobtained to have a weight-average molecular mass of greater than 80 000and a number-average molecular mass of greater than 20 000.

The process according to the invention can be carried out in thepresence of a solvent. The solvent is chosen in particular from benzene,toluene, xylene, ethyl acetate, methanol, ethanol, propanol,isopropanol, butanol, isobutanol, amyl alcohol, dimethyl sulphoxide,glycol, dimethylformamide, tetrahydrofuran and their mixtures, thesolvent/polymerization ingredients (namelymonomer(s)+initiator-controller) ratio by weight advantageously being atmost 5.

The process according to the present invention can also be carried outin the presence of a transfer agent in the usual amounts. The transferagents which can be used are well known to a person skilled in the artwho is an expert in the (co)polymerization of ethylene under highpressure. Mention may in particular be made of alkanes, for examplebutane, alkenes, for example propylene, and oxygen-comprisingderivatives, such as, for example, aldehydes or alcohols.

In accordance with the present invention, the ethylene can becopolymerized with any other monomer exhibiting a carbon-carbon doublebond capable of polymerizing or copolymerizing by the radical route.

The monomer or monomers can thus be chosen from vinyl, allyl,vinylidene, diene and olefinic monomers (other than ethylene).

The term “vinyl monomers” is understood to mean in particular(meth)acrylates, vinylaromatic monomers, vinyl esters, vinyl ethers,(meth)acrylonitrile, (meth)acrylamide and mono- and di(C₁-C₁₈ alkyl)(meth)acrylamides, and monoesters and diesters of maleic anhydride andof maleic acid.

The (meth)acrylates are in particular those of the formulaerespectively:

in which R⁰ is chosen from C₁-C₁₈ alkyl radicals of linear or branchedand primary, secondary or tertiary type, C₅-C₁₈ cycloalkyl radicals,(C₁-C₁₈)alkoxy(C₁-C₁₈)-alkyl radicals, (C₁-C₁₈)alkylthio(C₁-C₁₈)alkylradicals, aryl radicals and arylalkyl radicals, these radicalsoptionally being substituted by at least one halogen atom and/or atleast one hydroxyl group, the above alkyl groups being linear orbranched; and glycidyl, norbornyl or isobornyl (meth)acrylates.

Mention may be made, as examples of useful methacrylates, of methyl,ethyl, 2,2,2-trifluoroethyl, n-propyl, isopropyl, n-butyl, sec-butyl,tert-butyl, n-amyl, i-amyl, n-hexyl, 2-ethylhexyl, cyclohexyl, octyl,i-octyl, nonyl, decyl, lauryl, stearyl, phenyl, benzyl, β-hydroxyethyl,isobornyl, hydroxypropyl or hydroxybutyl methacrylates. Mention may bemade, in particular, of methyl methacrylate.

Mention may be made, as examples of acrylates of the above formula, ofmethyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl,hexyl, 2-ethylhexyl, isooctyl, 3,3,5-trimethylhexyl, nonyl, isodecyl,lauryl, octadecyl, cyclohexyl, phenyl, methoxymethyl, methoxyethyl,ethoxymethyl and ethoxyethyl acrylates.

The term “vinylaromatic monomer” is understood to mean, within themeaning of the present invention, an aromatic monomer comprisingethylenic unsaturation, such as styrene, vinyltoluene, α-methylstyrene,4-methylstyrene, 3-methylstyrene, 4-methoxystyrene,2-(hydroxymethyl)styrene, 4-ethylstyrene, 4-ethoxystyrene,3,4-dimethylstyrene, styrenes substituted on the ring by a halogen, suchas 2-chlorostyrene, 3-chlorostyrene, 4-chloro-3-methylstyrene,4-chloro-3-(tert-butyl)styrene, 2,4-dichlorostyrene and2,6-dichlorostyrene, 1-vinylnaphthalene and vinylanthracene.

Mention may be made, as vinyl esters, of vinyl acetate, vinylpropionate, vinyl chloride and vinyl fluoride and mention may be made,as vinyl ethers, of vinyl methyl ether and vinyl ethyl ether.

Mention is made, as vinylidene monomer, of vinylidene fluoride.

The term “diene monomer” is understood to mean a diene chosen fromconjugated or nonconjugated and linear or cyclic dienes, such as, forexample, butadiene, 2,3-dimethylbutadiene, isoprene, chloroprene,1,3-pentadiene, 1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene,1,9-decadiene, 5-methylene-2-norbornene, 5-vinyl-2-norbornene,2-alkyl-2,5-norbornadienes, 5-ethylene-2-norbornene,5-(2-propenyl)-2-norbornene, 5-(5-hexenyl)-2norbornene,1,5-cyclooctadiene, bicyclo[2.2.2]octa-2,5-diene, cyclopentadiene,4,7,8,9-tetrahydroindene, isopropylidenetetrahydroindene and piperylene.

Mention may be made, as olefinic monomers, of olefins comprising fromthree to twenty carbon atoms and in particular the a-olefins of thisgroup. Mention may be made, as olefin, of propylene, 1-butene,4-methyl-1-pentene, 1-octene, 1-hexene, isobutylene, 3-methyl-1-pentene,3-methyl-1-butene, 1-decene, 1-tetradecene or their mixtures.Fluorinated olefinic monomers may also be mentioned.

Mention may also be made, as (co)polymerizable monomers, of α- orβ-ethylenically unsaturated C₃₋₈ carboxylic acids, such as maleic acid,fumaric acid, itaconic acid, acrylic acid, methacrylic acid and crotonicacid; or α- or β-ethylenically unsaturated carboxylic acid anhydrides,such as maleic anhydride or itaconic anhydride.

Preferred comonomers are, inter alia, vinyl acetate, n-butyl acrylate,2-ethylhexyl acrylate, methyl acrylate and ethyl acrylate.

The process according to the invention is carried out in a tubularreactor or autoclave or a combination of the two.

Autoclave and tubular processes are both included among thepolymerization processes referred to as “high pressure” polymerizationprocesses known to a person skilled in the art. These two processesinvolve the polymerization of ethylene by the radical route under highpressure, generally between 100 and 350 MPa, and at temperatures greaterthan the melting temperature of the polyethylene being formed. Thetubular process involves polymerization in a tubular reactor. A tubularreactor comprises cylinders with an internal diameter generally ofbetween 1 and 10 cm and a length generally ranging from 0.1 to 3 km. Ina tubular reactor, the reaction mixture is driven with a high linearspeed, generally of greater than 2 metres per second, with shortreaction times which can, for example, be between 0.1 and 5 min.

The pressure in the tubular reactor can, for example, be between 200 and350 MPa, preferably between 210 to 280 MPa, for example between 230 and250 MPa. The temperature in the tubular reactor can range from 120 to350° C. and preferably from 150 to 300° C.

The autoclave process involves polymerization 20 in an autoclave with alength/diameter ratio generally ranging from 1 to 25, in the case of asingle-zone reactor. In the case of a multiple-zone reactor, the lengthof each zone/diameter ratio generally ranges from 0.5 to 6, it beingunderstood that the reaction mixture flows in the lengthwise direction.

The pressure in the autoclave reactor can, for example, be between 100and 250 MPa, preferably between 120 and 180 MPa, for example between 140and 170 MPa. The temperature in the autoclave reactor can range from 180to 300° C., preferably from 240 to 290° C.

The present invention also relates to block copolymers for which atleast one of the blocks comprises ethylene as constituent. As alreadyindicated above, these block copolymers are prepared by using themacroinitiators-controllers described above with reference to the secondembodiment.

Mention may in particular be made of the block copolymers of the type:

Polystyrene-(b) -polyethylene

Polyacrylate-(b)-polyethylene

Polymethacrylate-(b)-polyethylene

Poly(styrene-(co)-acrylate)-(b)-polyethylene

Polystyrene-(b)-poly(ethylene-(co)-acrylate)

Polystyrene-(b)-poly(ethylene-(co)-vinyl acetate),

it being possible for such block copolymers, as well as the homopolymersof ethylene and the random copolymers obtained according to theinvention, to have numerous applications as base for adhesives, forcoextrusion binders, for films, for bituminous binders, for packaging,for moulded items, and the like.

Comparative examples and nonlimiting examples describing the preparationof polymers as obtained according to the process of the presentinvention are given below. In these examples, the followingabbreviations were used:

BPO: benzoyl peroxide

DTBP: di-tert-butyl peroxide

TEMPO: 2,2,6,6-tetramethyl-1-piperidinyloxy

Hexyl-TEMPO:

CXA 5415: bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate:

(sold under the trade name “CXA 5415” by “Ciba Specialty Chemical”).

In these examples, the control of the polymerization was assessed usingthe curve ln(1/(1-conversion)) as a function of time. The more the curvedeviates from linearity, the poorer the control. Furthermore, inExamples 2 (comp.), 3, 4 (comp.) and 5, the mean rate of polymerization(expressed in min⁻¹) is the slope of the straight regression lineplotted from the ln(1/(1-conversion)) curve.

EXAMPLE 1 (COMPARATIVE) Uncontrolled Polymerization of Ethylene at 250MPa and at 250° C. with the BPO+TEMPO Initiating System

A metal reactor, made of steel which is resistant to high pressure, ispreheated using ring heaters to a temperature of 250° C. After rinsingthe reactor several times with ethylene at 6 MPa and then at 100 MPa,the reactor is charged with ethylene (17.6 g) and then, using a syringe,the following are injected into the reactor:

0.016 g of BPO (210 molar ppm of free radicals with respect to theethylene);

0.013 g of TEMPO; and

0.8 g of xylene.

The TEMPO/BPO molar ratio is 1.25, which corresponds to an SFR/Z ratioof 0.63 (i.e. a ratio of less than 1.0).

The pressure is subsequently increased to approximately 100 MPa, using amechanical pump and then to the desired value of 250 MPa, using a manualpump.

The conversion is measured continuously using an infrared spectrometerconnected to the reactor.

Runaway of the polymerization occurs, so much that no control of thepolymerization is possible.

This example emphasizes that an SFR/Z ratio<1 is insufficient to controlthe polymerization. Conversely, it is now well known that an SFR/Zratio>1 is unfavourable to the rate of polymerization (the excess ofstable radicals having a tendency to slow down the polymerizations).

The use of an initiator-controller introduces the advantage ofautomatically obtaining the optimum SFR/Z ratio of 1.0.

EXAMPLE 2 (COMPARATIVE) Polymerization of Ethylene at 170 MPa and at230° C. with the DTBP+TEMPO Initiating System

The polymerization is carried out as in Example 1, except that theethylene charge is 17.9 g, that the reaction is carried out at atemperature of 230° C. instead of 250° C. and at a pressure of 170 MPainstead of 250 MPa and that the BPO+TEMPO system is replaced by thefollowing system:

0.005 g of DTBP (106 molar ppm of free radicals with respect to theethylene); and

0.011 g of TEMPO.

The TEMPO/DTBP molar ratio is 2.0, which corresponds to an SFR/Z ratioof 1.

The following conversions over time are obtained:

Time (min) Conversion (%) ln(1/(1-conversion)) 0 0 0 15 2.7 0.027 30 8.50.089 45 15.2 0.165 60 21.2 0.238

The mean rate of polymerization is 0.0951 mint⁻¹.

EXAMPLE 3 Polymerization of Ethylene at 170 MPa and at 230° C. with thehexyl-TEMPO Initiator-controller

The operating conditions of Example 2 are used, except that theBPO+TEMPO system is replaced by the following “initator-controller”compound: 0.016 g of hexyl-TEMPO (104 molar ppm of free radicals withrespect to the ethylene). The SFR/Z ratio is 1.0.

The following conversions over time are obtained:

Time (min) Conversion (%) ln(1/(1-conversion)) 0 0 0 15 10.9 0.115 30 210.236 45 28.2 0.331 60 34.7 0.426

The rate of polymerization, which is here 0.1224 min⁻¹, is higher thanfor Comparative Example 2 (i.e. 23% more), while the operatingconditions are the same: pressure, temperature and amount of radicalswith respect to the ethylene. In addition, the curveln(1/(1-conversion)) as a function of time is a straight line, whichindicates good control of the polymerization.

Electron paramagnetic spectrometry measurements on hexyl-TEMPOfurthermore show that it is necessary to raise the temperature above200° C. to see the appearance of a signal. This means that the C—O bondonly cleaves from this temperature. In point of fact, the bond betweenthe polyethylene chain and the TEMPO, (—CH₂-TEMPO) is of the samenature. It is therefore necessary to heat above 200° C. to be able tocleave the bond between the TEMPO and the polymer. The conditions of theprocess of the present invention are therefore different from those ofthe process according to WO 99/03894, for which the temperature must notexceed 180° C. if control of the polymerization is not to be lost.

EXAMPLE 4 (COMPARATIVE) Polymerization of Ethylene at 200 MPa and 220°C. with the DTBP+CXA 5415 Initiating System

The operating conditions of Example 1 are used, except that the ethylenecharge is 18.3 g, that the reaction is carried out at a temperature of220° C. instead of 250° C. and at 200 MPa instead of 250 MPa and thatthe BPO+TEMPO system is replaced by the following system:

0.0045 g of DTBP (94 molar ppm of free radicals with respect to theethylene); and

0.016 g of CXA 5415.

The CXA 5415/DTBP molar ratio is 1.0, which is equivalent to an SFR/Zratio of 1.0.

The following conversions over time are obtained:

Time (min) Conversion (%) ln(1/(1-conversion)) 0 0 0 15 0.3 0.003 30 0.70.007 45 2.2 0.022 60 4.7 0.048

The polymerization is particularly slow (0.0008 min⁻¹): only 4.7%conversion is observed after one hour. Furthermore, the curveln(1/(1-conversion)) exhibits a marked curvature, which means that thepolymerization is not very well controlled.

EXAMPLE 5 Polymerization of Ethylene at 200 MPa and 220° C. with thehexyl-TEMPO Initiator-controller

The operating conditions of Comparative example 4 are used, except thatthe ethylene charge is 18.3 g and that 0.0157 g of hexyl-TEMPO (99 molarppm of free radicals with respect to the ethylene) is used instead ofthe CXA 5415/DTBP system.

The following conversions over time are obtained:

Time (min) Conversion (%) ln(1/(1-conversion)) 0 0 0 15 9.3 0.098 3013.3 0.143 45 16.8 0.184 60 20.4 0.228 75 27.1 0.316

The polymerization exhibits a higher rate of polymerization than forComparative Example 4, namely of 0.0038 min⁻¹ (i.e. 375% more than inComparative Example 4), while the operating conditions are identical:pressure, temperature and amount of radicals with respect to theethylene. At the end of one hour, the conversion has already reached20*t. Furthermore, the curve ln(1/(1-conversion)) as a function of timeis a straight line, which indicates good control of the polymerization.

Like Example 3, this example shows that hexyl-TEMPO makes possible moreefficient control of the polymerization of ethylene thaninitiator+stable radical mixtures and, furthermore, that it provideshigher rates of polymerization. EXAMPLE 6

Polymerization of Ethylene at 170 MPa and 250oC with the hexyl-TEMPORadical Initiator

The operating conditions of Example 1 are used, except that the ethylenecharge is 17.0 g and that 0.086 g of hexyl-TEMPO (587 molar ppm of freeradicals with respect to the ethylene) is used instead of the BPO+TEMPO)system.

The following conversions over time are obtained:

Time (min) Conversion (%) ln(1/(1-conversion)) 0 0 0 15 31.8 0.383 3052.9 0.753 45 62.1 0.970 60 68.2 1.146 75 72 1.273

Despite the severe conditions applied (large amount of radicals releaseddue to the large amount of hexyl-TEPO, and high temperature) thepolymerization remains controlled and does not exhibit runaway.Furthermore, the conversion is high. It would be impossible to carry outsuch a test without a stable radical as this would result in a verystrong runaway of polymerization.

What is claimed is:
 1. Process for the radical polymerization orcopolymerization of ethylene under high pressure in the presence of atleast one polymerization initiating-controlling compound capable ofproviding, by decomposition under the polymerization or copolymerizationconditions: at least one initiating free radical (Z) which carries atleast one site for initiating the (co) polymerization; and at least onestable free radical (SFR) which carries at least one site exhibiting thestable radical state and which is stable under the polymerizationconditions, with, in total, as many initiating sites as sites exhibitingthe stable radical state.
 2. Process according to claim 1, wherein thechoice is made, as initiating-controlling compound, of a compoundcapable of providing at least one monofunctional radical Z chosen fromthose of the formulae (Ia) or (Ib) or (Ic):

in which: R¹, R², R³, R⁴, R⁵, R⁶ and R⁷ each independently represent:optionally substituted C₁₋₂₄ alkyl; or optionally substituted C₅₋₂₄aryl; and it also being possible for R¹, R², R³, R⁴, R⁵ and R⁶ to eachindependently denote a hydrogen atom.
 3. Process according to claim 1,wherein the choice is made, as initiating-controlling compound, of acompound capable of providing at least one monofunctional radical Zchosen from those of the formulae (Ia₁), (Ia₂) or (Ia₃):

with n=0 or an integer from 1 to 23;

with R⁸ representing hydrogen, methyl or ethyl; and

with: R⁹, R¹⁰, R¹¹, R¹² and R¹³ each independently representing alkyl,aryl or halogen; and R¹⁴ representing alkyl or aryl.
 4. Processaccording to claim 1, wherein the choice is made, asinitiating-controlling compound, of a compound capable of providing aradical Z of formula (II):  Z¹—(PM)¹—[(PM)²]^(•)  (II) in which: Z¹represents the initiating fragment of a radical initiator; (PM)1represents a polymer block formed by living radical polymerization orcopolymerization of at least one monomer which can polymerize by theradical route in the presence of an initiator which produces freeradicals Z^(1•); and (PM)², the presence of which is optional,represents another polymer block, other than (PM)¹, formed by livingradical polymerization or copolymerization of at least one monomer whichcan polymerize by the radical route in the presence of the initiatorZ¹—(PM)^(1•).
 5. Process according to claim 4, wherein the (PM)¹ and(PM)² blocks have the formula:

where: T and U each independently represent hydrogen or a substituted orunsubstituted C₁₋₁₀ alkyl residue; V and W each independently representhydrogen, substituted or unsubstituted alkyl, substituted orunsubstituted aryl, —COOH, —COOR¹⁵, —CN, —CONH₂, —CONHR¹⁶, —CONR¹⁷R¹⁸,

or OR²⁰, R¹⁵ to R²⁰ each independently representing substituted orunsubstituted alkyl or substituted or unsubstituted aryl; and n denotesthe degree of polymerization.
 6. Process according to claim 1, whereinthe choice is made, as initiating-controlling compound, of a compoundcapable of providing a polyfunctional radical Z carrying a plurality ofinitiating sites of

type.
 7. Process according to claim 6, wherein the choice is made, asinitiating-controlling compound, of a compound capable of providing apolyfunctional radical Z of formula:


8. Process according to one of claims 1 to 7, wherein the choice ismade, as initiating-controlling compound, of a compound capable ofproviding at least one nitroxyl stable free radical comprising at leastone═N—O• group.
 9. Process according to claim 8, wherein the choice ismade, as initiating-controlling compound, of a compound capable ofproviding at least one nitroxyl stable free radical chosen from those ofthe formulae (IIIa), (IIIb) or (IIIc):

in which: R′¹ to R′³, R′⁵to R′⁸and R′¹³ and R′¹⁴ each independentlyrepresent: (a) a hydrogen atom; (b) a halogen atom; (c) a saturated orunsaturated and linear, branched or mono- or polycyclichydro-carbonaceous group which can be substituted by at least onehalogen; (d) an ester group —COOR′¹⁵ or an alkoxyl group —OR′¹⁶, R′¹⁵and R′¹⁶ representing a hydrocarbonaceous group as defined in point (c)above; or (e) a polymer chain; R′⁴ has the meanings defined in points(a), (b), (c), (d) and (e) above; R′⁹to R′¹², which are identical ordifferent, have the meanings defined in points (a) to (e) above and canin addition represent a hydroxyl group or an acid group, such as —COOHor —SO₃H; it being possible for R′³ and R′⁴to be connected to oneanother and, in the case where R′⁴ represents a —CR″¹R″²R″³ residue (R″¹to R″³ having without distinction the meanings of R′¹ to R′³), it beingpossible for R′³to be connected to R″³, to form a heterocycle comprisingthe nitrogen atom of

it being possible for the said heterocycle to be saturated orunsaturated, to comprise, in the ring, at least one other heteroatomand/or at least one

group and also to comprise a saturated or unsaturated fused ring; itbeing possible for two from R′¹ to R′³, R′⁵and R′⁶, R′⁷ and R′⁸, R′⁹ andR′¹⁰, R′¹¹ and R′¹², R′⁶ and R′⁹, R′⁸ and R′¹¹, or R′₁₃ and R′¹⁴ and, inthe case where R′⁴ represents a —CR″¹R″²R″³ residue, R′³ and R″3independently to be connected to one another to form, with the carbonatom which carries them, a saturated or unsaturated ring or heterocycle;and u is a non zero integer.
 10. Process according to claim 1, whereinthe choice is made, as initiating-controlling compound, of a compoundcapable of providing at least one nitroxyl stable free radical chosenfrom:

where: R^(a) to R ^(f) have independently the meanings given for R′⁹ toR′¹², it being possible for R^(a) and R^(b) and R^(e) and R^(t) to beidentical or different when they are carried by different carbon atoms;r has the value 2or 3or 4; s is a non zero integer; and t has the value0, 1 or
 2. 11. Process according to claim 9, wherein the choice is made,as initiating-controlling compound, of a compound capable of providingat least one nitroxide stable radical chosen from:2,2,5,5-tetramethyl-1-pyrrolidinyloxy;3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy;2,2,6,6-tetramethyl-1-piperidinyloxy;4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy;4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy;4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy; bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate;N-tert-butyl-1-phenyl-2-methylpropyl nitroxide; andN-tert-butyl-1-(2-naphthyl)-2-methylpropyl nitroxide.
 12. Processaccording to claim 1, wherein an alkoxyamine is used asinitiating-controlling compound.
 13. Process according to claim 1,wherein use is made, as initiating-controlling compound, of analkoxyamine having a nitroxide, the nitroxide does not decompose to morethan 50% over 2 hours at 180° C. under 200 MPa heptane.
 14. Processaccording to claim 1, wherein use is made, as initiating-controllingcompound, of a compound chosen from:


15. Process according to claim 1, wherein the ratio of theinitiating-controlling compound/monomer(s) is within the range from0.0001% to 10% by weight.
 16. Process according to claim 15, wherein theratio of the initiating-controlling compound/monomer(s) is within therange from 0.0005% to 5% by weight.
 17. Process according to claim 1,wherein it is carried out under a pressure of 150 to 300 MPa. 18.Process according to claim 17, wherein it is carried out under apressure of 150 to 250 MPa.
 19. Process according to claim 1, wherein itis carried out at a temperature of 100 to 300° C.
 20. Process accordingto claim 19, wherein it is carried out at a temperature of 180 to 250°C.
 21. Process according to claim 1 for the polymerization of ethylene,wherein it is carried out at a temperature, a pressure and a durationwhich are sufficient for the polyethylene obtained to have aweight-average molecular mass of greater than 80000 and a number-averagemolecular mass of greater than
 20000. 22. Process according to claim 1,wherein it is carried out in the presence of a solvent.
 23. Processaccording to claim 22, wherein the solvent is chosen from benzene,toluene, xylene, ethyl acetate, methanol, ethanol, propanol,isopropanol, butanol, isobutanol, amyl alcohol, dimethyl sulphoxide,glycol, dimethyl-formamide, tetrahydrofuran and their mixtures. 24.Process according to claim 22, wherein the solvent/ polymerizationingredients ratio by weight is at most
 5. 25. Process according to claim1, wherein it is carried out in the presence of a transfer agent. 26.Process according to claim 1, wherein it comprises a copolymerization ofethylene with at least one comonomer chosen from vinyl, allyl,vinylidene, diene and olefinic monomers.
 27. Process according to claim1, wherein it is carried out in a tubular reactor or autoclave or acombination of the two.
 28. Process according to claim 5, wherein nranges up to 10,000.
 29. Process according to claim 8, wherein thechoice is made, as the initiafing-controlling compound, of a compound ofthe following formulae:

wherein: R^(g), R^(h), R^(i), R^(j), R^(k) and R^(m) each independentlyrepresent: (a) a hydrogen atom, (b) a halogen atom, (c) a saturated orunsaturated and linear, branched or mono- or polycyclic hydrocarboneousgroup which can be substituted by at least one halogen, (d) an estergroup —COOR′¹⁵ or an alkoxyl group —OR′¹⁶, R′¹⁵ and R′¹⁶ representing ahydrocarbonaceous group as defined in (c) above, (e) a polymer chain,(f) a hydroxyl group, or (g) an acid group such as —COOH or —SO₃H; andR′¹, R′², R″¹ and R″² each independently represent (a), (b), (c), (d) or(e) above.
 30. Block copolymers obtained by controlled radicalpolymerization or copolymerization of ethylene under high pressure inthe presence of at least one polymerization initiating-controllingcompound having provided, by decomposition under the polymerization orcopolymerization conditions: at least one initiating free radical (Z)which carries one site for initiating the (co)polvmerization and whichhas the formula (II):

in which: Z¹ represents the initiating fragment of a radical initiator;(PM)¹ represents a Polymer block formed by living radical polymerizationor copolymerization of at least one monomer which can polymerize by theradical route in the presence of an initiator which produces freeradicals Z^(1•); and (PM)², the presence of which is optional,represents another polymer block, other than (PM)¹, formed by livingradical polymerization or copolymerization of at least one monomer whichcan polymerize by the radical route in the presence of theinitiator—Z¹—(PM)^(1•), and at least one stable free radical whichcarries one site exhibiting the stable free radical state and which isstable under the polymerization conditions.
 31. Block copolymeraccording to claim 28, wherein each (PM)¹ block and each (PM)² block hasthe formula:

where: T and U each independently represent hydrogen or a substituted orunsubstituted C₁₋₁₀ alkyl residue; V and W each independently representhydrogen, substituted or unsubstituted alkyl, substituted orunsubstituted aryl, —COOH, —COOR¹⁵, —CN, —CONH₂, —CONHR¹⁶, —CONR¹⁷R¹⁸,

or OR²⁰, R¹⁵ to R²⁰ each independently representing substituted orunsubstituted alkyl or substituted or unsubstituted aryl; and n denotesthe degree of polymerization.